C# Visitor

The Visitor design pattern represents an operation to be performed on the elements of an object structure. This pattern lets you define a new operation without changing the classes of the elements on which it operates.

Frequency of use:
low
C# Design Patterns

UML class diagram

A visualization of the classes and objects participating in this pattern.


Participants

The classes and objects participating in this pattern include:

  • Visitor  (Visitor)
    • declares a Visit operation for each class of ConcreteElement in the object structure. The operation's name and signature identifies the class that sends the Visit request to the visitor. That lets the visitor determine the concrete class of the element being visited. Then the visitor can access the elements directly through its particular interface
  • ConcreteVisitor  (IncomeVisitor, VacationVisitor)
    • implements each operation declared by Visitor. Each operation implements a fragment of the algorithm defined for the corresponding class or object in the structure. ConcreteVisitor provides the context for the algorithm and stores its local state. This state often accumulates results during the traversal of the structure.
  • Element  (Element)
    • defines an Accept operation that takes a visitor as an argument.
  • ConcreteElement  (Employee)
    • implements an Accept operation that takes a visitor as an argument
  • ObjectStructure  (Employees)
    • can enumerate its elements
    • may provide a high-level interface to allow the visitor to visit its elements
    • may either be a Composite (pattern) or a collection such as a list or a set

Structural code in C#

This structural code demonstrates the Visitor pattern in which an object traverses an object structure and performs the same operation on each node in this structure. Different visitor objects define different operations.

using System;
using System.Collections.Generic;

namespace Visitor.Structural
{
    /// <summary>
    /// Visitor Design Pattern
    /// </summary>

    public class Program
    {
        public static void Main(string[] args)
        {
            // Setup structure

            ObjectStructure o = new ObjectStructure();
            o.Attach(new ConcreteElementA());
            o.Attach(new ConcreteElementB());

            // Create visitor objects

            ConcreteVisitor1 v1 = new ConcreteVisitor1();
            ConcreteVisitor2 v2 = new ConcreteVisitor2();

            // Structure accepting visitors

            o.Accept(v1);
            o.Accept(v2);

            // Wait for user

            Console.ReadKey();
        }
    }

    /// <summary>
    /// The 'Visitor' abstract class
    /// </summary>

    public abstract class Visitor
    {
        public abstract void VisitConcreteElementA(
            ConcreteElementA concreteElementA);
        public abstract void VisitConcreteElementB(
            ConcreteElementB concreteElementB);
    }

    /// <summary>
    /// A 'ConcreteVisitor' class
    /// </summary>

    public class ConcreteVisitor1 : Visitor
    {
        public override void VisitConcreteElementA(
            ConcreteElementA concreteElementA)
        {
            Console.WriteLine("{0} visited by {1}",
                concreteElementA.GetType().Name, this.GetType().Name);
        }

        public override void VisitConcreteElementB(
            ConcreteElementB concreteElementB)
        {
            Console.WriteLine("{0} visited by {1}",
                concreteElementB.GetType().Name, this.GetType().Name);
        }
    }

    /// <summary>
    /// A 'ConcreteVisitor' class
    /// </summary>

    public class ConcreteVisitor2 : Visitor
    {
        public override void VisitConcreteElementA(
            ConcreteElementA concreteElementA)
        {
            Console.WriteLine("{0} visited by {1}",
                concreteElementA.GetType().Name, this.GetType().Name);
        }

        public override void VisitConcreteElementB(
            ConcreteElementB concreteElementB)
        {
            Console.WriteLine("{0} visited by {1}",
                concreteElementB.GetType().Name, this.GetType().Name);
        }
    }

    /// <summary>
    /// The 'Element' abstract class
    /// </summary>

    public abstract class Element
    {
        public abstract void Accept(Visitor visitor);
    }

    /// <summary>
    /// A 'ConcreteElement' class
    /// </summary>

    public class ConcreteElementA : Element
    {
        public override void Accept(Visitor visitor)
        {
            visitor.VisitConcreteElementA(this);
        }

        public void OperationA()
        {
        }
    }

    /// <summary>
    /// A 'ConcreteElement' class
    /// </summary>

    public class ConcreteElementB : Element
    {
        public override void Accept(Visitor visitor)
        {
            visitor.VisitConcreteElementB(this);
        }

        public void OperationB()
        {
        }
    }

    /// <summary>
    /// The 'ObjectStructure' class
    /// </summary>

    public class ObjectStructure
    {
        List<Element> elements = new List<Element>();

        public void Attach(Element element)
        {
            elements.Add(element);
        }

        public void Detach(Element element)
        {
            elements.Remove(element);
        }

        public void Accept(Visitor visitor)
        {
            foreach (Element element in elements)
            {
                element.Accept(visitor);
            }
        }
    }
}
Output
ConcreteElementA visited by ConcreteVisitor1
ConcreteElementB visited by ConcreteVisitor1
ConcreteElementA visited by ConcreteVisitor2
ConcreteElementB visited by ConcreteVisitor2

Real-world code in C#

This real-world code demonstrates the Visitor pattern in which two objects traverse a list of Employees and performs the same operation on each Employee. The two visitor objects define different operations -- one adjusts vacation days and the other income.

using System;
using System.Collections.Generic;

namespace Visitor.RealWorld
{
    /// <summary>
    /// Visitor Design Pattern
    /// </summary>

    public class Program
    {
        public static void Main(string[] args)
        {
            // Setup employee collection

            Employees employee = new Employees();
            employee.Attach(new Clerk());
            employee.Attach(new Director());
            employee.Attach(new President());

            // Employees are 'visited'

            employee.Accept(new IncomeVisitor());
            employee.Accept(new VacationVisitor());

            // Wait for user

            Console.ReadKey();
        }
    }

    /// <summary>
    /// The 'Visitor' interface
    /// </summary>

    public interface IVisitor
    {
        void Visit(Element element);
    }

    /// <summary>
    /// A 'ConcreteVisitor' class
    /// </summary>

    public class IncomeVisitor : IVisitor
    {
        public void Visit(Element element)
        {
            Employee employee = element as Employee;

            // Provide 10% pay raise

            employee.Income *= 1.10;

            Console.WriteLine("{0} {1}'s new income: {2:C}",
                employee.GetType().Name, employee.Name,
                employee.Income);
        }
    }

    /// <summary>
    /// A 'ConcreteVisitor' class
    /// </summary>

    public class VacationVisitor : IVisitor
    {
        public void Visit(Element element)
        {
            Employee employee = element as Employee;

            // Provide 3 extra vacation days

            employee.VacationDays += 3;

            Console.WriteLine("{0} {1}'s new vacation days: {2}",
                employee.GetType().Name, employee.Name,
                employee.VacationDays);
        }
    }

    /// <summary>
    /// The 'Element' abstract class
    /// </summary>

    public abstract class Element
    {
        public abstract void Accept(IVisitor visitor);
    }

    /// <summary>
    /// The 'ConcreteElement' class
    /// </summary>

    public class Employee : Element
    {
        private string name;
        private double income;
        private int vacationDays;

        // Constructor

        public Employee(string name, double income,
            int vacationDays)
        {
            this.name = name;
            this.income = income;
            this.vacationDays = vacationDays;
        }

        public string Name
        {
            get { return name; }
            set { name = value; }
        }

        public double Income
        {
            get { return income; }
            set { income = value; }
        }

        public int VacationDays
        {
            get { return vacationDays; }
            set { vacationDays = value; }
        }

        public override void Accept(IVisitor visitor)
        {
            visitor.Visit(this);
        }
    }

    /// <summary>
    /// The 'ObjectStructure' class
    /// </summary>

    public class Employees
    {
        private List<Employee> employees = new List<Employee>();

        public void Attach(Employee employee)
        {
            employees.Add(employee);
        }

        public void Detach(Employee employee)
        {
            employees.Remove(employee);
        }

        public void Accept(IVisitor visitor)
        {
            foreach (Employee employee in employees)
            {
                employee.Accept(visitor);
            }
            Console.WriteLine();
        }
    }

    // Three employee types

    public class Clerk : Employee
    {
        // Constructor

        public Clerk()
            : base("Kevin", 25000.0, 14)
        {
        }
    }

    public class Director : Employee
    {
        // Constructor
        public Director()
            : base("Elly", 35000.0, 16)
        {
        }
    }

    public class President : Employee
    {
        // Constructor
        public President()
            : base("Eric", 45000.0, 21)
        {
        }
    }
}
Output
Clerk Kevin's new income: $27,500.00
Director Elly's new income: $38,500.00
President Eric's new income: $49,500.00

Clerk Kevin's new vacation days: 17
Director Elly's new vacation days: 19
President Eric's new vacation days: 24

.NET Optimized code in C#

The .NET optimized code demonstrates the same real-world situation as above but uses modern, built-in .NET features, such as, generics, reflection, LINQ, lambda functions, etc. You can find an example on our Singleton pattern page.

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